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Mold (US) or mould (UK) are multicellular fungi that grow in the form of filaments called hyphae that appear like long branch-like structures under a microscope. Although not a taxonomic designation, there are an estimated 10,000 - 300,000 of species of molds that live in a wide variety of environments [1]. In general they cause biodegredation of foods and other natural materials (such as wood in buildings), which has a wide role of effects depending on the species of mold. For example, some species of mold can be the cause of food spoilage, and other species play an important role in food production such as in cheese and soy sauce production [2]. Certain species of mold are thought to contribute precursors that effect flavor production in the toasting of oak barrels [3] Molds have also been used in the pharmaceutical industry to produce antibiotics and other medicines. Certain molds can cause allergic reactions as well as diseases from mold growth within the body or from mycotoxin production in exposed foods or from breathing spores from mold growth in buildings [2].

Mold in Beer and Health Concerns

Mold Growth in Beer

Although little studying has been done on mold growth in beer, it has been known to occur in barrel aged beers are fruited sour beers. Oxygen generally needs to be present in order for mold to grow. Mold growth on fruit that is in sour beer can occur if the fruit is floating above the surface of the beer, and enough oxygen is present (either from vacuum suck-back, oxygen permeability of the vessel, or sampling). For this reason, many commercial sour brewers will "punch down" the fruit frequently with a dish-like tool.

Preventing Mold Growth

  • If aging sour beer on whole fruit, use a punch-down tool frequently so that the low pH kills any mold that might be present before it begins to grow. CO2 may be helping the fruit to float, and punching-down the fruit will help to release that CO2. Homebrewers can gently swirl their carboys to accomplish the same thing.
  • Limit oxygen exposure to the beer.
  • Use diligent sanitation practices.
  • Inspect barrels for mold prior to use; discard barrels that have mold growth, or disassemble the barrel and vigorously clean/sanitize the wood before use. See the Barrel page for details on storing and maintaining barrels.
  • Mold is tolerant of both hops and alcohol levels above what is in beer (50-60% ABV), so do not rely on hops or alcohol to prevent mold growth [4].
  • Keep relative humidity levels under 60% and preferably between 30-50%. This level of relative humidity also falls into the preferred levels for oak barrel environments [5].

Health Concerns

A small number of molds can produce poisonous substances called mycotoxins, or cancer-causing poisons called aflatoxins. Some molds that do not produce toxins can still cause allergic reactions or respiratory problems [1]. Their effects can be accumulative, rather than immediately toxic as in the case of pathogenic bacteria. As a general rule of thumb, pigmented molds (non-white/grey) are more likely to produce poisonous substances (especially black mold which could be Aspergillus), however, there are exceptions to this guideline [6]. Some mycotoxins can survive boiling temperatures in wort to varying degrees, so if mold develops during processes such as kettle souring, it is possible for some percentage of mycotoxins to survive the wort boiling process and fermentation (one study showed that 20-30% of two mycotoxins were removed during boiling and fermentation) [7]. Only mold that is growing on the beer itself is considered potentially dangerous; mold growing in airlocks or outside of vessels should not be a cause for concern as long as they do not come into contact with the beer itself.

Identifying mold based on what it looks like is not a reliable way to determine if a mold is a cause of concern for health [6]. If mold is present in beer, it might be possible for commercial breweries to remove the mold and send it to a lab to check if it is a toxic mold before allowing the beer to be consumed. Some homebrewers may decide to risk taking a chance that the mold will not cause illness, however, we recommend discarding beer that has come into contact with mold, especially if serving to others. Mold growing on soft fruits and vegetables with high moisture content can be contaminated below the surface, therefore beer or wort could also be contaminated even if the mold was just growing only on the surface [1].

See the USDA website, this UC Davis Food safety sheet, and "Fact or Fiction? Can Pathogens Survive in Beer - Mould Edition.," by Bryan of Sui Generis Blog for more information on the health concerns of mold that grows on food.

Identifying Mold

As mentioned above, identifying the species of mold based on what it looks like with the naked eye is not a reliable way to identify it at the species level, and thus not a reliable way to know if the mold growth is safe. It is, however, usually easy to tell the difference between general mold growth and pellicle or yeast growth. Mold tends to look "fuzzy" or "hairy" and often has green, red, black, white, or grey colors, where as yeast activity or pellicles tend to have evidence of CO2 production (bubbles), and looks "creamy", "powdery", or "chalky" and is never green, red, or black in color. Protein/carbohydrate coagulation can also be misidentified as mold, however this also never appears "hairy" or "fuzzy" like mold. We recommend taking a picture through a hand lens when posting an image of what might be mold as this will make it easier to identify. Be sure that the color of the mold/pellicle in the photo is accurate to what it looks like with the naked eye (sometimes cameras make white substances appear more pigmented, which can throw off attempts to identify the organism online). Please make an attempt to identify whether the growth is mold or not on your own before posting in the group (these types of posts clog the stream!). See the images below for examples that should help.

Mold vs Pellicle Examples

The following table shows examples of mold or pellicles from the Milk The Funk Facebook group, and the consensus on whether mold is likely present or not based on the image.

Mold or Pellicle? Determination
Mold ex1.jpg Pellicle or yeast fermentation activity, not mold.
Mold ex2.jpg Undetermined, but the brewer shook the carboy and the mass went away, so probably not mold.
Mold ex3.jpg Undetermined; the small green area in the middle of the pellicle might be mold.
Mold ex4.jpg Occurred in a clean dubbel after adding prunes; pellicle, not mold.
Mold ex5.jpg Mold growth during kettle souring (Lactobacillus was the only microbe pitched into the wort).
Mold ex6.jpg Mold growth on top of blackberries.
Mold ex7.jpg Small amount of pellicle, not mold.
Mold ex8.jpg Green and fuzzy mold.
Mold ex9.jpg Mold growth on an oak barrel bung. Beer is ok if the mold is scrubbed and removed; recommended to use rubber or silicon for the bung instead.
Mold ex10.jpg Pellicle on top of green grapes, not mold.
Mold ex11.jpg Mold growth on fruit that is floating above the surface of the beer.
Mold ex12.jpg Partial pellicle on a kettle sour; probably a wild yeast contamination from the added fruits. Not mold.
Mold ex13.jpg Mold growth from under-pitching with only one bottle of commercial beer dregs.
Mold ex14.jpg Brettanomyces activity, not mold.
Mold ex15.jpg Probably mold or another potentially unwanted microorganism from an attempt at collecting wild yeast from dried camomile.
Mold ex16.jpg Partial pellicle, not mold.
Mold ex17.jpg Pellicle, not mold.
Mold ex22.jpg Fermentation from a turbid mashed spontaneous inoculation; not mold.
Mold ex18.jpg Image of mold through a hand lens; photo taken with an iPhone by Mark Trent.
Mold ex19.jpg Identified in a lab to be Penicillium (considered a mold).
Mold ex20.jpg Mold in a barrel that made the beer taste like soy sauce; potentially Aspergillus (some can be carcinogenic).
Mold ex21.jpg Mold growth on an agar plate from swabbing persimmon in an attempt to catch wild yeast; probably not a dangerous mold.
Mold ex23.jpg Mold growth during kettle souring. Possibly Penicillium or Aspergillus; neither are good.

See Also

Additional Articles on MTF Wiki

External Resources